1. Field of the Invention
The present invention relates to a printed circuit board and a printed wiring board each of which has a constitution that receiving circuits are electrically connected to a main wiring through respective branch wirings.
2. Description of the Related Art
Generally, a memory system has been constituted so as to have a memory controller and a plurality of memory devices. Here, “DDR3-SDRAM” (Double Data Rate 3-Synchronous Dynamic Random Access Memory) has been generally known as the memory device.
Incidentally, there are two mounting configurations of the memory devices, i.e., one configuration in which the memory devices are mounted on a motherboard, and the other configuration in which the memory devices are mounted on a module substrate. In the configuration in which the memory devices are mounted on the module substrate, the module substrate having the mounted memory devices thereon is connected through a connector to a motherboard on which a memory controller has been mounted, thereby constituting a memory system.
The memory controller transmits an address signal or a command signal (or an address/command signal), and each of the memory devices, which receives the address/command signal, is controlled based on the received signal, so that data signals are transmitted and received between the memory controller and the plurality of memory devices. Under such a condition, particularly in a high-functional electronic device, the plurality of DDR3-SDRAMs are often mounted and used as the memory devices to secure a sufficient memory capacity.
Each of the memory devices, which is realized by the DDR3-SDRAM, has a built-in function for adjusting signal transmission timing. The respective memory devices are connected to a universal-shape main wiring which is called a fly-by-wire and enables high-speed transmission of the address/command signal.
Incidentally, in the JEDEC standard (No. 21C PC3-6400/PC3-8500/PC3-10600/PC3-12800 DDR3 Unbuffered SO-DIMM Reference Design Specification), the inner two layers in a printed wiring board are used as the main wiring. Moreover, a BGA (Ball Grid Array) semiconductor package is used as the memory device. On the printed wiring board, a via hole formed on the main wiring, a mounting pad connected to the receiving terminal of the BGA semiconductor package, and an extraction wiring for connecting the via hole and the mounting pad to each other are formed so as to together constitute the branch wiring for branching from the main wiring to the memory device. Further, when the small-diameter via hole capable of being arranged between the mounting pads is used as a part of the branch wiring, the branch wiring to the memory device can be shortened.
On the other hand, in Japanese Patent Application Laid-Open No. 2009-86841, it has been proposed, in a DIMM (Dual Inline Memory Module) having a DDR3-SDRAM mounted on a module substrate, to reduce a ringing of a waveform by adjusting the length of a branch wiring on the DIMM.
However, in the constitution of the above module substrate, the length of the branch wiring has been defined to be short, i.e., 3 mm or so, according to the DIMM standard. For this reason, conventionally, the wiring length of the extraction wiring is shortened by arranging the via hole adjacently to the mounting pad, thereby resultingly shortening the wiring length of the branch wiring. However, in the event that a large number of bus wirings such as address command wirings or the like are provided and the mounting pads are arranged at high density, there is a case where the via hole cannot be arranged between the mounting pads and is thus arranged outside the group of the mounting pads. In that case, it is necessary to prolong the wiring length of the branch wiring. As the wiring length of the branch wiring becomes long, problems of signal attenuation, and signal reflection become serious, thereby causing turbulence of signal waveforms. In other words, it causes ringing of signals. In particular, a problem concerning the waveform of the address/command signal in the DDR3-SDRAM is that there occurs a case that the ringing of the signal becomes large and thus an input voltage condition of the signal cannot be satisfied.
In consideration of such disadvantages as described above, the present invention aims to provide a printed circuit board and a printed wiring board which can suppress the ringing of the waveform of the signal received by the receiving circuit irrespective of the wiring length of the branch wiring.